1. Field of the Invention
This invention relates to a hub clutch used as a four-wheel drive device.
2. Description of the Related Art
There is a conventional automatic free-wheel hub clutch (hereinafter referred to simply as "hub clutch") which has been disclosed in U.S. Pat. No. 4,327,821 and which comprises a drive gear engaging a drive shaft and provided with clutching splines as well as other splines on the periphery thereof, a clutch ring slidably engaging splines extending on the inner circumference of a body in the axial direction thereof and having a clutchingspline on the inner circumference thereof, a return spring for inwardly urging the clutch ring and stretched between a clutch cap (cover) and said clutch ring in the axial direction (clutch-off direction), a cam follower disposed adjacent the clutch ring and engaged with the other splines on the periphery of said drive shaft at the inner circumference thereof in a transferable manner with respect to the axial direction and further provided with an inverted V-shaped projection projecting in the axial direction as well as protrusions (teeth) formed on the periphery of said inverted V-shaped projection at the extreme end thereof and protruding radially outward therefrom, a cam member having a V-shaped grooved cam surface for mating with and seating the inverted V-shaped projection of said cam follower and secured to a stationary system (locking nut), a movable cam provided coaxially around the cam member in a rotatable manner and having a substantially inverted V-shaped convex cam surface extending in the axial direction as well as a cam stop on the extreme end of said cam surface, and a drag shoe for braking said movable cam with said stationary system. A thrust washer is disposed between said drag shoe and said lock nut, and the hub clutch is constructed to withstand a pressig force transferred from the return spring to the clutch ring, the cam follower, the movable cam, and the drag shoe. A shift spring is coaxially mounted inside of said return spring and extends in the axial direction between the clutch cap and the drive gear. Said clutch ring and said cam follower are in face-to-face contact in a slidable manner.
In the above-described clutch hub, when the drive shaft beings to rotate, the drive gear rotates integrally there-with, so that torque is transmitted from said other splines to the cam follower. The inverted V-shaped projection of the cam follower which is disposed in the V-shaped grooved cam surfce of the cam member moves outwardly in the axial direction along said V-shaped grooved cam surface, the inverted V-shaped projection is moved out of said V-shaped groove, and then the projection engages with the inverted V-shaped convex cam surface of the movable cam (the rotation of which is suppressed by the drag shoe) and moves axially and outwardly along the cam surface. As the clutching splines on the periphery of the drive gear engage the clutch ring, the engagement or locking thereof becomes stronger with an increase in the outward movement of the projection in the axial direction along the convex cam surface of the movable cam, and when the protrusions engage the cam stop, the locking is completed. The maintenance of such a locked condition is facilitated by braking the movable cam with the drag shoe. Furthermore, both the clutching splines and the other splines are aligned on the drive shaft adjacent each other in an axially transferable condition, and the shift spring stretched between the clutching spline (positioned outside in the axial direction0 and the clutch cap limits the outward displacement of the clutch ring in the axial direction.
Moreover, occasionally a vehicle must be maneuvered by repeating forward and backward movements in a completely meshed four-wheel drive (4WD) mode. In this case, since the drive shaft is rotated in a direction reverse to that in which the vehicle had just travelled, the cam follower is also rotated in the reverse direction, and the inverted V-shaped projection of the cam follower drops into the groove of the V-shaped grooved cam surface in the cam member. As a result, the clutch ring is pushed inwardly by means of an expandig force exerted by the return spring in the axial direction, whereby the clutching splines are disengaged.
In the above-described hub clutch, various disadvantages arise because the clutching splines become disengaged when the vehicle repeats forward and backward movements. For example, when a vehicle travelling in 4WD during a steep ascent comes to the end of a rough road, the vehicle must repeat forward and backward movements by turning the wheel in order to make a U-turn. As a result, there arises either a free position state or a poor engagement of the clutching spline so that the vehicle travels in an unlocked or incompletely locked condition. In such a situation, the vehicle begins moving in the two-wheel drive (2WD) condition by means of only the driving wheels. Thus, such a situation is particularly disadvantageous on a snow-covered road, a muddy road, in off-road conditions and the like.
Furthermore, when the vehicle travels in four-wheel drive, the drag shoe continually slides on a stationary bolt, so that a problem occurs in that the wear is accelerated and durability of the drag shoe decreses remarkably.